Already known in the prior art is a three-way catalytic converter which can clean three major toxic components (HC, CO and NO.sub.x) in the exhaust gas. The operation of the three-way catalytic converter is effectively attained when the exhaust gas is in a state wherein excess air as well as excess fuel is not left in the exhaust gas, i.e., the air-fuel ratio of the exhaust gas (the ratio of air remaining in the exhaust gas to fuel remaining in the exhaust gas) is maintained near the stoichiometric value.
For maintaining the air-fuel ratio near the stoichiometric value, an air injection system has been provided in the prior art for increasing or decreasing the amount of secondary air introduced into the exhaust manifold in accordance with electrical signals transmitted from an oxygen concentration-cell type air-fuel sensor, for example, an O.sub.2 sensor which is arranged in an exhaust pipe of the engine.
However, this system has a disadvantage in that the air-fuel ratio cannot be effectively maintained near the stoichiometric value since the O.sub.2 sensor cannot correctly detect the air-fuel ratio of the exhaust. Due to the fact that the air-fuel ratio is not maintained near the stoichiometric value, the three-way catalytic converter cannot operate effectively. As a result, a considerable amount of toxic components will remain in the gas exhausted from the engine into the atmosphere.